1. Field of the Invention
The present invention relates to a coating apparatus and, more particularly, to an improved, extrusion-type coating apparatus having a doctor edge which enables a coating solution to be continuously extruded toward a surface of a running support so that a coating of uniform thickness is applied to the support.
2. Description of the Prior Art
The term "support", as used herein, means a flexible, beltlike material which generally has a width between 0.3 and 3.0 mm, a length between 45 and 20,000 mm, and a thickness between 2 and 20 .mu.m--e.g., films made of plastics such as polyethylene terephthalate, polyethylene-2, 6-naphthalate, cellulose diacetate, cellulose triacetate, cellulose acetate propionate, polyvinyl chloride, polyvinyllidene chloride, polycarbonate, polyimide, and polyamide; paper; paper with polymers of .alpha.-olefins containing from 2 to 10 carbon atoms such as polyethylene, polypropylene, and an ethylene-butene copolymer which is coated or laminated; and metallic foils of aluminum, copper, tin, etc. It also includes a belt-like material comprising the above-described belt-like material (as a substrate) provided with a preliminary processing layer.
The type of material chosen as the coating solution depends on the purpose for which the ultimate coated material is to be used, e.g., a photographic, light-sensitive coating solution; a magnetic coating solution; and a surface protecting, antistatic or lubricating coating solution. After the desired coating solution is coated onto a surface of the support, the support is dried and cut to the desired dimensions. Typical examples of such coated materials include photographic films, printing papers, and magnetic tapes.
Conventional, doctor edge-equipped extruders which are intended to apply a uniform coating of a coating solution on a support are described in Japanese Patent Application (OPI) No. 138036/75 (the term "OPI", as used herein, means a "published unexamined Japanese patent application") and Japanese Patent Publication No. 7306/79. In addition, a further, improved extruder is described in Japanese Patent Application No. 159899/80.
The common disadvantage of conventional, doctor edge-equipped extruders is that the range within which coating is possible is very limited. In particular, when coating at a rate of 100 to 150 m/min or more, it is very difficult to stably coat a coating solution which has a thickness of 20 .mu.m or less in the form of a liquid using any of the conventional extruders.
Extensive investigations have revealed that the above-described problem which arises when using the conventional extruders is created as follows:
The running support is responsible for the introduction of air into the extruder portion. This phenomenon becomes more markedly abrupt at coating rates of 100 to 150 m/min or more. Therefore, in uniformly coating a thin film within the above range, it is important that the liquid pressure at the outlet of a slot be controlled appropriately. If the liquid pressure is low, a coating film which has an uneven thickness is formed by the introduction of air bubbles and/or scrubbing of the coating solution, i.e., the counterflow of the coating solution toward the upper stream. On the other hand, if the extruder produces only a high liquid pressure when the amount of the coating solution is small, the thickness in the widthwise direction of the resulting coating film is likely to become uneven.